The present invention relates generally to anti-skid systems for automotive vehicles. Means have been provided in the past for detecting the relative slowdown of a vehicle wheel which occurs in a skid to produce a skid signal and for utilizing such signal to relieve braking pressure at the particular wheel. Such anti-skid systems having predominantly utilized electronically generated signals. The means for generating and responding to such signals have tended to be complex and costly. Such systems have suffered from a general lack of reliability, and some of the systems have been susceptible to running out of fluid as a result of repeated cycling occurring within a sustained stroke of the brake pedal. Resort has been had to using air as the signalling medium, but air systems are equally unreliable and disadvantageous for other reasons.